1. Technical Field of the Invention
This invention relates to CVD reactors and to methods for chemical vapor deposition of bulk polysilicon directly onto the walls of the reaction chamber and on tube walls within the chamber. More particularly, it relates to production of bulk polysilicon by a chemical vapor deposition process where a removable thin wall tubular casing is used to construct the reaction chamber within a CVD reactor, and where additional removable middle and core tubes may also be employed; the tube walls providing additional surface area upon which the polysilicon is deposited, external and core heat sources providing a flatter thermal gradient and improved thermal efficiency.
2. Background Art
One of the widely practiced conventional methods of polysilicon production is by depositing polysilicon in a chemical vapor deposition (CVD) reactor, and is referred to as Siemens method. In this method, polysilicon is deposited in a CVD reactor on high-purity thin silicon rods called xe2x80x9cslim rodsxe2x80x9d. Because of the high purity silicon from which these slim rods are fabricated, the corresponding electrical resistance of the slim rods is extremely high. Thus it is extremely difficult to heat this silicon xe2x80x9cfilamentxe2x80x9d using electric current, during the startup phase of the process.
Sometimes the slim rods are replaced by metallic rods that are more conductive and easier to heat with electrical current. This method is referred to as Rogers Heitz method. However, the introduction of metal into the chemical vapor deposition process introduces metal contamination. This contamination of the polysilicon yield is not acceptable in the semiconductor/microelectronics industry.
In the Siemens method, external heaters are used to raise the temperature of these high purity rods to approximately 400xc2x0 C. (centigrade) in order to reduce their electrical resisitivity. Sometimes external heating is applied in form of halogen heating or plasma discharge heating. However in a typical method, to accelerate the heating process, a very high voltage, in the order of thousands of volts, is applied to the rods. Under the very high voltage, a small current starts to flow in the slim rods. This initial flowing current generates heat in the slim rods, reducing the electrical resistance of the rods and permitting yet higher current flow and more heat.
This process of sending low current at high voltage continues until the temperature of slim rods reaches about 800xc2x0 C. At this temperature, the resistance of the high purity silicon rods falls very drastically and the high voltage source is switched to a low voltage source that is capable of supplying high current.
Referring to prior art FIG. 1, a CVD reactor consists of a base plate 23, quartz bell jar 17, chamber cover 24, bell jar supports 16, and heater 18 between the bell jar and the chamber cover. There is incorporated in base plate 23, a gas inlet 20 and a gas outlet 21, and electrical feedthroughs 19. A viewing port 22 provides for visual inspection of the interior.
In the prior art polysilicon manufacturing process by CVD, the silicon slim rod structure is assembled in the form of a hair pin by having a cross rod 2 placed horizontally on two long, spaced apart, vertical rods 1 and 3. The structure is mounted and connected so as to provide a current path between electrical feedthroughs 19, generating the heat necessary for deposition to occur. During the CVD process, polysilicon deposit accumulates uniformly on the slim rods; the deposit being shown here partially removed to show the slim rod structure. Deposits of silicon on the reactor walls can occur if they become hot enough, so cooling of the reactor walls is sometimes employed to prevent this.
Different users employ different methods for joining the horizontal rod to the vertical rods. One method requires a groove or a key slot at the top of each vertical rod. A small counter bore or conforming fitment is formed on the ends of the horizontal rod so that it can be press fitted into the grooves to bridge the two vertical rods.
A typical prior art reactor consists of a complex array of subsystems. Two power sources are required, one power supply that can provide very high voltages and low current; and a second power supply that can sustain a very high current at relatively lower voltage. Also needed are the slim rod heaters and their corresponding power supply for preheating the slim rods. Another component is the high voltage switch gear. Moreover, the entire startup process is cumbersome and time consuming. Since the current drawn by the slim rods at around 800xc2x0 C. is of a run away nature, the switching of the high voltage to low voltage needs to be done with extreme care and caution.
Also, through this electric current method for heating the slim rods, the rods become an interior heat source losing tremendous amounts of heat via radiation to the surroundings. There is significant energy loss inherent in the existing practice.
There is a plethora of prior art in the general area of reactors used for chemical vapor deposition, some employing intentional deposition on heated reactor walls. For example, Jewett""s U.S. Pat. No. 4,265,859 is a system for producing molten polycrystalline silicon and replenishing the melt of a crystal growth crucible. The system includes a hot wall, muffle furnace reactor in which silicon is deposited in low density form on the wall and inner tube of the reaction chamber by delivering a gaseous silicon compound through the heated chamber, at nominally 1000 degrees Centigrade. After a certain amount of silicon has been deposited on the fused quartz chamber walls and inner tube of the reactor, the chamber temperature is raised higher, to about 1450 degrees Centigrade, to melt down the silicon deposit for recovery, letting it run molten out the bottom of the reactor into the melt crucible of the crystal growth part of the system. When the heat is reduced in the outflow trap, a silicon plug forms, re-sealing the reactor for the next cycle. This gas inflow/molten silicon outflow reactor operation is repeated cyclically without cooling or opening the reactor between cycles, to support the crystal growth operation. The hot quartz chamber wall of the Jewett reactor requires a fully encircling support, disclosed as graphite, to sustain wall integrity at high temperature. It is not used or useful for the production of bulk polysilicon ingots.
Gautreaux et al""s U.S. Pat. No. 4,981,102 discloses a hot wall CVD muffle furnace reactor with a heated liner for collecting silicon deposits on the inner face, from a through-flow of silicon gas. The reactor can be cycled to high heat to melt the silicon for molten outflow, or opened via a large door on the reactor to remove the liner so that the deposited silicon can be removed from the inside surface of the liner for use as bulk ingots of polycrystalline silicon. The liner is disclosed to be a removable unitary or assemblage of liner components, fabricated or coated with molybdenum, graphite, silicon, silicon nitride or other materials. It is not known to be in commercial practice.
Jewett""s U.S. Pat. No. 4,123,989 discloses a horizontal muffle furnace and method for producing silicon by CVD on the inside of a silicon tube emplaced horizontally in the through-flow furnace so as to define the reaction chamber. The silicon tube is reportedly supported on its sidewall within the muffle tube by graphite support rings, and sealed or at least supported securely for alignment at both ends to cooling head end caps through which the process materials are flowed. Water is circulated through the cooling heads to prevent deposition on the cooling heads. A muffle tube of quartz or other high temperature, non-contaminating material surrounds the silicon tube. The space between the muffle tube and the silicon tube may be held at an overpressure state with argon to assure no out gassing from the silicon tube chamber. A resistance heater system surrounds the muffle tube, and the assemblage is insulated from without. Jewett describes the thin wall silicon tube as a product of EFG (Edge-defined Film fed Growth) process.
There are inherent problems and limitations with the Jewett disclosure, published in November, 1978. It is inherently a single tube, through-flow, deposition process. Assembly is complex, with water cooled end caps, plumbing and alignment issues. Adequate support for the horizontally oriented, thin wall tube at high temperature is very difficult to achieve, without farther altering heat distribution and deposition patterns with more support rings. To the Applicant""s knowledge, the device has never been placed in commercial service. Further, no production method in use commercially is known to the Applicant to employ external heat to a reactor as the primary source of heat for CVD of bulk polysilicon.
Examples of tubular wall reactors for the cyclic deposition and melting of silicon include Tokuyama Corp""s JP8259211, issued Oct. 8, 1996, disclosing a decomposition/reduction reactor for silanes and production of crystalline silicon; and Wacker Chemitronic""s DE4127819, a reactor using a silicon tube heated by direct conduction of an electric current for deposition and melting of the silicon.
Other patents are more clearly distinguished but may provide some additional context to the general field of reactive vessels and processes. Massey et al""s EP0164928, is a vertical hot wall CVD reactor for thin film deposition on substrates, utilizing a bell jar on a platform envelope within the furnace. Gas inlet and exhaust manifolds extending upward from the base plate and having ports spaced along their length, combined with the geometry of the substrate carrier assembly, provide generally separate, virgin gas flow patterns across each of the stacked substrates. This device is not designed, used for, or obviously adaptable to the production of bulk polysilicon. McBrayer, Jr., et al""s U.S. Pat. No. 5,552,039 is a turbulent flow, cold-wall reactor for supercritical water processes combined with corrosive atmospheres, using a bell jar/base plate design with a vertical core feeding tube extending into the reaction zone.
The broad goal of the invention is to introduce improvements in the means for generating ingots of polysilicon by a chemical vapor deposition (CVD) process. The most common carrier gas for chemical vapor deposition of polysilicon is hydrogen, although an inert gas could be used. Suitable reactant materials for use with a hydrogen carrier are either silane, SiH4, having a deposition temperature in the order of 800 degrees centigrade, or any of the chlorosilanes, which have a deposition temperature in the order of 1000 to 1200 degrees, depending on the actual composition and process details. The gaseous byproducts of the CVD process reactor are removed continuously through outlet ports of the reactor, as new carrier gas and reactant materials are admitted through inlet ports into the reactor. The supply, reclamation and recycling of process materials outside the reactor is outside the scope of this disclosure.
It is an object of the invention to provide an apparatus and method for more efficient production of polysilicon by chemical vapor deposition, using less power and resulting in lower cost, by using a consumable, removable, thin wall tubular casing to construct a one time use, vertically oriented reaction chamber within a cold wall reactor envelope, heating the reaction chamber radiantly from outside the envelop of the reactor, flowing and counter flowing silicon ladened gas in the reaction chamber, and depositing polysilicon directly upon the interior wall of the tubular casing.
In accordance with the invention, a cold wall chemical vapor deposition reactor is specifically designed to utilize a reaction chamber constructed from a consumable, replaceable component of the system, a section of tubular material, which may be of silicon, graphite or suitable metals and is preferably an EFG thin wall silicon tube. The chamber tube is supported on a base support, preferably graphite. The chamber and base support are closed at bottom and top ends as may be done with a base plate and cover plate respectively. The inner surface of the tube becomes the wall of the reaction chamber. Slight over pressure may be maintained through the base plate to a blanket zone between the envelope of the reactor and the reaction chamber to protect from outgassing.
When radiant heat is applied from sources external to the chamber sufficient to heat the chamber walls to the necessary deposition temperature, as with jacket heaters applying radiant heat through a cold wall quartz envelope onto the silicon chamber tube, and a select combination of carrier gas and reactant materials containing silicon are admitted into the chamber through the base plate to be flowed and counter-flowed about the chamber and exhausted through the base plate, the hot chamber walls then becoming a broad surface area available for deposition of bulk polysilicon.
As the chemical vapor deposition proceeds, a continuous, broad surface layer of polysilicon is deposited on the chamber wall, building in thickness so that the interior diameter of the chamber grows progressively smaller. This results in the production of a hollow, tubular bulk quantity of polysilicon within a thin wall tubular casing. Upon completion of the deposition cycle, the reactor is disassembled sufficiently far so as to enable the end product, the chamber tube and its deposit, to be removed. A new chamber tube is installed for the next deposition cycle. The tube or casing can be included or removed from the end product prior to further processing of the polysilicon.
Study of flow patterns in the capped vertical chamber show that the flow of process gases between the base plate inlet and outlet extends throughout the chamber to all exposed tube walls, providing flow and counter flow movement in the chamber that assures generally fall and uniform deposition over the length of the chamber, and promotes a more efficient process, generally extracting and depositing more silicon per volume of input gases than comparable through flow reactors.
It is a another object of the invention to increase the initial deposition surface area over that of a basic vertical tube wall deposition chamber. To that end, there is provided a further refinement of the invention, the placement of a consumable, replaceable, somewhat smaller diameter thin wall middle tube, preferably an EFG silicon tube, in the center of the reaction chamber, supported on spaced apart or ventilated graphite supports and not extending fully to the chamber top so as to permit circulation of gases over, under and around the middle tube, so that broad area deposition occurs on both the inner and outer surfaces of the middle tube concurrently with deposition on the inner surface of the shell or chamber tube. This results in a larger yield over the same reaction time, and more efficient use of the reactor. Upon completion of the deposition cycle, the middle tube with its inner and outer deposit layers is likewise harvested as a product of the invention.
It is yet another object of the invention to improve the efficiency of the heating system and reduce the heat losses, relative to the yield of polysilicon. It is a further object of the invention to reduce the temperature gradient from the exterior to the interior of the reactor of the invention so as to promote a greater and more uniform rate of deposit on all available surface area. It is a yet further object to provide more initial surface area for deposition. To these ends, a as different or additional heating element is incorporated into the device and methodology previously described. A co-axial heater configured with an external deposition surface area core tube is placed at the axial center of the primary reaction chamber of a single tube or multi-tube embodiment of the invention, or as a sole deposition surface in a cold wall CVD reactor, extending upwards from the base plate. An elongate radiant heating element is axially positioned and supported within the heater tube, with suitable connections to external sources of power and control through the base plate, and is sealed directly or indirectly from the deposition process.
The process is conducted as previously described, except that heat is preferably applied to the system from the external radiant heater assembly and the core heating element, thereby heating the primary reaction chamber and all deposition surfaces from both sides to a more uniform temperature, resulting in a faster startup and more uniform deposit rate. When the deposition has been carried to the desired extent, the apparatus is deconstructed and the heater tube with its exterior deposit of polysilicon is added to the total yield of the batch. Reloading the reactor will, of course, require a new heater tube assembly or a new tubular deposition cover tube over the heater element.
Still other objectives and advantages of the present invention will become readly apparent to those skilled in this art from the detailed description, wherein we have shown and described a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention.